

The plants on the lakes so monitor the water temp so they don’t affect the ecosystem during the warmer seasons still.
Yeah, but look at the magnitudes of the heat units involved. Modern nuclear plants generate 0.6-4.5 GW at around 30% thermal efficiency (so they generate between 2-15GW of heat). These underwater data centers are looking at 25 MW (0.025 GW) while surrounded by water in 5 of the 6 3-dimensional directions.
There is some risk to local ecosystems, but we’re literally talking 2 or more orders of magnitude difference compared to nuclear plants or other thermal plants.
I think you have to look at the actual orders of magnitude difference in raising the temperature of water versus air. The Arizona story you linked is about a study that found up to +4°F (+2.2°C) temperatures in air.
The same amount of heat, spread across the same volume of water moving at the same speeds, would only raise that water by
1/830 as much, for a +0.0048°F (+0.0027°C)1/3300 as much, for a +0.0012°F/+0.00067°C temperature change across the same area/volume.(I got to 830 by taking the specific heat of dry air of approx 1 J/g K at room temperature and regular atmospheric pressure and 1.22 kg/m^3, versus water’s 4.184 J/g K and 1000 kg/m^3).(Edit: I fucked my math. Water has approximately 3300 times the heat capacity as air, per unit volume, and I just looked it up directly).
The higher conductivity of water might be offset by the higher convection potential of air (because air responds to temperature changes with differences in density/pressure, which creates wind in itself), so that the heat will spread through either medium relatively quickly and therefore dissipate very quickly with distance to the source.
I just don’t see a world where a data center raises the water by even 1°C, even locally.